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  • 1.
    Tuomas, Roger
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Isaksson, Ove
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Measurement of lubrication conditions in a rolling element bearing in a refrigerant environment2009Ingår i: Industrial Lubrication and Tribology, ISSN 0036-8792, E-ISSN 1758-5775, Vol. 61, nr 2-3, s. 91-99Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Purpose - The ability to build up a film thickness separating the contacts in the bearing is essential to assure long service life of rolling element bearings. Rolling element bearings used in refrigeration suffer from poor film thickness due to decreased viscosity of the lubricant by the dilution of refrigerant in the oil. The purpose of this paper is to redesign a bearing test apparatus equipped with a capacitance measuring device able to monitor the lubrication status in the bearing online and include experiments to verify the capacitance measuring technique. Design/methodology/approach - The objective is to design a complete system to study film build up in rolling element bearings in a refrigerant environment and to use and evaluate the capacitance/resistance measuring technique.Findings - The investigation shows good correlation between denting on tested bearings and the identified contacts by the capacitance measuring apparatus. The method is also useful when studying lift-off or run-in of a bearing.Originality/value - in this paper, a bearing test apparatus is redesigned and equipped with a capacitance measuring device

  • 2.
    Tuomas, Roger
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Isaksson, Ove
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    The effect of phosphate additives on the lubrication of rolling element bearings in a refrigerant environment2007Ingår i: International journal of refrigeration, ISSN 0140-7007, E-ISSN 1879-2081, Vol. 30, nr 1, s. 28-36Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Chlorine free replacement refrigerants, HFC (hydrofluorocarbons) and HC (hydrocarbons), have shown less lubricating properties. Phosphate based additives were used to improve the lubricity with refrigerant R-134a, demonstrating positive effects. In the present paper, the ability to form lubricating film and wear of two additives, phosphate ester and acid phosphate, was investigated in a bearing test apparatus. The results show that phosphate additive in polyolester oil, in an R-134a environment, increases the lubricating film and reduce wear. Surface topography during the initial run-in changes to a more favorable profile with lower RMS angle and longer wavelengths that promote load-carrying capacity and film build-up.

  • 3.
    Tuomas, Roger
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Isaksson, Ove
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Compressibility of oil/refrigerant lubricants in elasto-hydrodynamic contacts2006Ingår i: Journal of tribology, ISSN 0742-4787, E-ISSN 1528-8897, Vol. 128, nr 1, s. 218-220Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A high-pressure chamber is used to study lubricant compressibility when refrigeration oil is diluted by refrigerant. The tested lubricant in this work is a POE (polyol ester) oil, POE diluted with nonchlorinated (HFC) refrigerant R-134a, a naphthenic mineral oil, and the mineral oil diluted with the chlorinated (HCFC) refrigerant R-22. The high-pressure chamber experiments show that by adding 20 wt% of R-134a to the polyol ester oil, the stiffness of the lubricant increases by approximately 38 wt% at 1 GPa and is much higher than for R-22 and mineral oil.

  • 4.
    Tuomas, Roger
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Properties of oil and refrigerant mixtures: lubrication of ball bearings in refrigeration compressors2006Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    A refrigeration compressor contains highly loaded mechanical contacts that have to be lubricated. The lubricant used in refrigeration compressors consists of an oil and refrigerant mixture, and the refrigerant concentration can be up to 40 wt%. However, the oil in the refrigeration system is a contaminant that lowers the efficiency of the refrigeration system. Therefore, it is important that the oil and the refrigerant are miscible with each other so that oil following the refrigerant out in the system is returned to the compressor and not accumulated in the system. HFC refrigerants are not miscible with the mineral oils used in CFC refrigeration systems. Hence, new synthetic ester lubricants were developed for this application. It was soon evident, though, that the lack of chlorine and that the new ester oils did not have the same good lubrication properties as mineral oil and CFC refrigerants. The objective of this thesis is to increase the understanding of lubricating rolling element bearings in refrigeration compressors. The thesis will also give recommendations for lubrication of bearing in refrigeration compressors based on the results. The thesis is based on an experimental study of parameters that are important for life of rolling element bearings. The thesis consists of five papers that describe a test apparatus designed to measure and monitor the lubrication status during operation with capacitance. The effect of two phosphate additives in refrigeration oil is also examined and properties such as viscosity, pressure-viscosity coefficient, compressibility and shear strength coefficient of oil and refrigerant mixtures are measured. The general conclusion of the thesis is that the refrigerant affects all of the measured properties negatively on the subject of lubrication and life of the rolling element bearing. The usage of oil-free compressors solves many of the problems that lubricants consisting of a mixture of oil and refrigerant cause. If the oil has to be in the system it is of great importance that the lubrication issues are taken care of early in the design process. The design parameters for temperature, viscosity, pressure- viscosity coefficient, contamination etc. must be followed when the compressor is in use.

  • 5. Tuomas, Roger
    Bearing lubrication of refrigeration screw compressors2001Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    The bearings in a modern refrigeration screw compressor are lubricated with a mixture of oil and refrigerant. However, little or no published bearing life data is available for the new generation non-chlorinated refrigerants. The work presented in this thesis concerns the development of a measuring technique and experimental equipment for bearing life studies. The equipment is intended to provide data about bearings lubricated with mixtures of oil/refrigerant for use by compressor designers. Bearing life is affected by the working lubricant's ability to form a film to separate the contact surfaces. To provide a sufficient film thickness in an elastohydrodynamic lubricated (EHL) contact, the viscosity, h, and pressure-viscosity coefficient, a, of the lubricant both play an important role. The film thickness in an EHL contact lubricated with an oil-refrigerant mixture with increasing amounts of refrigerant and different load ratios have been measured experimentally. The lubricant mixture tested consisted of a VG68 polyolester refrigeration oil, Solest 68, with R-134a as refrigerant. To measure the film thickness, an on-line capacitance method, Lubcheck, developed by SKF was used. The refrigerant dilution was increased until the lubricating film broke down and asperity contact occurred. The refrigerant's influence on the rheological properties of the oil was measured in a high pressure Höppler viscometer. R-134a, R-32, R-410a and R-22 refrigerant were mixed with a VG68 polyolester oil, Mobil 68 Arctic. Measurements showed that the viscosity and pressure-viscosity coefficient decrease with increasing dilution by the refrigerant. The work showed that film formation in contacts lubricated with oil/refrigerant mixtures is more sensitive to load than other investigations have indicated. A test procedure has been developed to determine the refrigerant concentration at which metal-to-metal contact occurs. The results also showed that run-in behaviour also appears in bearings used in refrigeration applications.

  • 6. Tuomas, Roger
    et al.
    Almqvist, Torbjörn
    Åhrström, Bert-Olof
    Berg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Influence of molecular structure on the lubrication properties of four different esters2000Ingår i: Tribologia, ISSN 0780-2285, Vol. 19, nr 4, s. 3-8Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The lack of published data on the chemical structures of lubricants makes it almost impossible to investigate the influence of structure on lubrication properties. In this investigation, the lubricating properties of three esters with known chemical structure have been investigated and compared with a commercial ester. The lubrication properties that were expected to be dependent on chemical structure such as film thickness and traction, viscosity and friction coefficients were compared by experiment. To measure the film thickness a Ball and Disc Apparatus was used, the traction coefficient was measured in a Jumping Ball Apparatus, the viscosity in a rotational cylindrical viscometer and the friction coefficient in a reciprocating friction and wear test apparatus. The results showed that molecular length has a significant influence on lubrication properties, with longer molecules giving the highest viscosity and greatest film thickness. The length of the molecule did not influence the coefficients of friction, but the traction coefficient, Υ, decreased with increasing molecular length.

  • 7. Tuomas, Roger
    et al.
    Jonsson, Ulf
    Luleå tekniska universitet.
    Influence of refrigerant on viscosity and pressure-viscosity coefficient of refrigeration compressor lubricants2000Ingår i: Thinning films and tribological interfaces: proceedings of the 26th Leeds-Lyon Symposium on Tribology held in the Institute of Tribology, School of Mechanical Engineering, The University of Leeds, UK 14th - 17th September, 1999 / [ed] D. Dowson, Amsterdam: Elsevier, 2000, s. 419-423Konferensbidrag (Refereegranskat)
    Abstract [en]

    Following the introduction of non-chlorinated HFC (hydrofluorocarbon) refrigerants and polyolester lubricants, designers of refrigeration compressors have had to re-evaluate their knowledge of compressor bearing lubrication. The higher solubility and the lack of anti wear protection in comparison to the traditional refrigerant/lubricant mixes makes the lubrication of bearings using modern systems a much more difficult task than before. A modern twin screw compressor contains several highly loaded oil lubricated, metal-to-metal including, rolling element bearings, gears and the rotors. These types of contacts are normally referred as EHD or EHL contacts (Elastohydrodynamic-Lubrication), due to elastic deformation of the surfaces. The expected life of these contacts is closely related to the lubricant film thickness. The lubricant properties that determine the film thickness in an EHD-contact include the dynamic viscosity, η, and the pressure-viscosity coefficient, α. In a refrigeration system, both those parameters are affected by the presence of refrigerant, the actual effect depending on the proportions of oil and refrigerant. A falling ball viscometer has been used to measure the viscosity and hence determine the pressure-viscosity coefficient as a function of the amount of dissolved refrigerant. A polyolester oil was tested with refrigerants R-22, R-134a, R-410a and R-32 at 40° and 80°C, at refrigerant concentrations from 0 % to 30 % dilution. It has been shown that the Eyring equation can be used predict the pressure-viscosity coefficient of a mixture based on the molecular weight ratio of the refrigerants.

  • 8. Krantz, Joakim
    et al.
    Tuomas, Roger
    Bhushan, Bharat
    Electrorheological Fluids for Lubrication1999Ingår i: Lubrication engineering, ISSN 0024-7154, Vol. 55, nr 3, s. 28-35Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Electrorheology (ER) is the phenomenon in which the rheology of a mixture of oil and particles is modified by the imposition of electric field. Conventional lubricants with cornstarch are shown to exhibit ER effect. Addition of cornstarch to the lubricant slightly increases the value of coefficient of friction, but is still acceptable for lubrication. Thus it appears that these ER fluids can also be used for lubrication, which may be desirable in some machine applications in which the lubricant is made more viscous to reduce leakage of the lubricant when the machine is not in use. Some lubricants mixed with cornstarch are tested to study the influence of concentration of cornstarch, oil properties and the strength and gap thickness of the electric field on the friction properties. Various experiments are performed to explain what happens in the mixture when the field is applied, how the mixture acts as a lubricant and how the composition of the mixture may influence the ER effect. This study shows that the particles orient in the mixture and because of that, the mixture becomes more viscous when the field strength and the concentration of cornstarch are increased. It is also observed that the viscosity decreases with an increasing gap thickness. An increase in concentration also increases the viscosity of the mixture even with no applied field and the lubrication effect goes down. In all cases, the viscosity levels off after the voltage is high enough and then does not increase with an increase in voltage. ER fluids can offer many exciting applications.

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